1. Field of the Invention
The present invention relates to a system for and a method of making a channel time reservation as part of a media access control in a distributed wireless personal area network, which is based on a wireless mobile ad-hoc network.
2. Description of the Related Art
A wireless personal area network is defined as a network working within a personalized area of about 10 meters. IEEE (Institute of Electrical and Electronics Engineers) prescribes standards for such a wireless personal area network. An ultra wide band (UWB) communication technology can provide a transmission rate of over several hundreds mega bits per second (Mbps) in this personalized area. In such wireless personal area network, the wireless media is shared among all the devices for communication. Therefore, it requires a media access method for controlling the access to the media by the devices. In a broad sense of meaning, it includes how to access the network, how to transmit data to other devices at a desired transmission rate, and how to use the media optimally.
A media access control for the wireless personal area network can be designed in two access methods, i.e., a centralized access mode and a distributed access mode. In the centralized access, one of the devices acts as a coordinator for the whole network in order to manage and coordinate a media access for all the devices. All the devices request assistance from the centralized coordinator for a media access such as participation to the network or allocation of channel time. In the distributed access, the media access is uniformly distributed through all the devices of the network. In addition, all the devices share the burden of managing media access with each other.
FIG. 1 shows one example of a conventional wireless personal area network with centralized coordinator.
The network shown in FIG. 1 includes a centralized network called a ‘piconet,’ while supporting a centralized media access mode based on IEEE802.15.3. One device, which is called a PNC (Piconet Coordinator) operates as a coordinator in the piconet. A PNC, such as PNC 10 of FIG. 1, allows other devices to be connected to the network, and provides a function of allocation and synchronization of the channel (time slot), through which data is transmitted to other devices. This is an ad-hoc centralized wireless personal area network.
FIG. 2 shows a wireless personal area network having no centralized coordinator.
Referring to FIG. 2, a plurality of devices are included and marked by a dot. Circles made about each device indicate the communication range for each device.
The network of FIG. 2 supports a distributed media access control mode. Each device cooperates with the other devices, allows a new device to participate in the network, and shares information required for performing a media access control, such as channel time allocation and synchronization for transmitting data to other devices, and electric power saving. Therefore, no device in the network acts as a dedicated coordinator. This is a distributed ad-hoc WPAN system.
The distributed media access control mode relies on a timing concept called ‘superframe.’ The superframe has a fixed length in time, and is divided into a plurality of time windows called a ‘time slot.’ In addition, the time slot is also called a ‘media access slot (MAS).’ Some of the time slots are used for the devices to send a beacon. The remaining slots are used to send data. The slot where beacons are sent may be called a ‘beacon slot’ and the slot where data is sent may be called a ‘data slot.’ The length of a beacon period (BP) may be smaller than that of data period. The beacon slots may be distributed over the slots of a superframe, or they may appear together in the starting portion of a superframe. Furthermore, the number of beacons and beacon slots may be fixed and may vary.
FIG. 3 shows the structure of a conventional superframe.
The superframe structure shown in FIG. 3 is basically defined in the Multiband OFDM (Orthogonal Frequency Division Modulation) scheme. This includes 256 media access slots (including a11, a12, and a21). Some of the media access slots (including a11 and a12) constitute a beacon period a10, which is constructed of beacon slots. The remaining media access slots (including a21) form a data section a20 constituted of media access slots, which can be used by other devices of the network in order to transmit data to other devices in the network.
Each media access slot (including a11, a12, and a21) forms a superframe having a length of approximately 65 ms, and each media access slot (including a11, a12, and a21) is 256 μs.
Information of the superframe is broadcast within a beacon, which is broadcast by each device in the beacon period. Thus, neighbor devices of that device can use the information for a subsequent processing. The start time of a superframe is determined by the start-up of a beacon period, and defines a beacon period start time (BPST).
A device must find a free slot in the beacon slot in order to send a beacon. The device, which sends its own beacon periodically, is considered as part of the network. Furthermore, devices need a free data slot for communication with another device. In order to reserve such a data slot, a source device and a receiver device must know that a particular data slot is free. The reservation of data slot is carried out in a completely distributed manner in the devices, which share information and assist in the slot reservation with each other. Dissimilar to the centralized WPAN, it is noted here that none of the devices are operated as a central coordinator for various media access tasks.
Once a beacon slot is reserved, it is used by the device for sending a beacon as long as the device is part of the network. If required, a device may change the slot of sending beacon during its operation. On the contrary, data slots become free if devices stop using them. Such free data slots are added to a free data slot pool, and may be reserved for other devices. A device cannot reserve a slot already reserved by another device.
A conventional system cannot detect and solve a reservation conflict proactively, which may occur between two devices wanting to reserve a same data slot at the same time. Consequently, there is a need for an efficient and reliable method of performing a slot reservation in the distributed wireless personal area network.